2012 Audi S8

With a new twin-turbo V-8, the 520-bhp S8 is more fuel-efficient than ever.

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Audi and Bentley have developed a new V-8, a compact twin-turbo 4.0-liter that can automatically switch over into fuel-saving 4-cylinder mode. The only discernible visual difference between the 2012 Audi S8s fitted with the new engine and the old models are four tiny microphones for the car's Active Noise Control system, plus a horizontal bar graph between the tach and speedo to indicating to the driver when the large sedan is running in V-4 or V-8 mode.

I drove Bentley's Mulsanne with its cylinder deactivation system last year, so I was intrigued to see how Audi's faired. Bentley doesn't bother with a graph telling the driver he's in eco mode, and I frankly found Audi's slightly distracting.

After a while, I suppose you either get used to it or just ignore the green bar flashing left and right as the engine dips in and out of V-4/V-8 mode. But, on a brief 12- mile drive through Ingolstadt's outskirts, I was consciously hunting for that switchover spot between V-8 and V-4 and found it frustratingly hard to maintain a green V-4 setting. Going by my Bentley experience I think high-speed freeway cruising would find you slipping easily into V-4 mode as the miles cruised by.

In the Bentley, I could not tell if its 6.75-liter V-8 was running on all eight or just four cylinders. The same isn't quite true for the S8's 4.-liter; there's a subtle change in exhaust beat as the engine dips from eight to four cylinders; maybe I was more aware of it because I kept searching for that switchover point. To ensure smooth running as a V-4, switchable electronic active engine mounts are used on an Audi for the first time, while Active Noise Control in the cabin disguises any intrusive engine noise.

I will reserve my final judgment on the S8 until I have spent more time driving the engine. One thing is for sure: with VW announcing a similar system for its 4-cylinder engines, cylinder deactivation is here to stay.

For the record, the 4.0-liter V-8 will see duty in Audi's S6, S6 Avant, S7 Sportback, with 420 bhp. In the S8, that power is increased to 520. This V-8 will also appear in a unique Bentley Continental GT scheduled to be revealed at the Detroit show in January.

Audi says the cylinder deactivation reduces fuel consumption in the European NEDC test cycle, by about 5 percent. Also noteworthy: At 50 mph, Audi says all S models with this engine have a 12 percent gain in efficiency. At 80 mph, the fuel savings is said to be 7 percent. Michael Dick, Audi's head of technical development, says American drivers might see fuel savings as high as 11.5 percent compared with the outgoing engines.